What I have found is that nearly all of the sealed mint box eBay
purchases have had sub-par, highly bag marked coins, while the ones
purchased directly from the mint are, on average, much nicer. The
eBay purchases have been through a lot of different sellers.
Obviously, the mint boxes are not 'high-security', nor is there any
method of detecting those which have been opened carefully and then
re-sealed with glue.
Does anyone actually buy sets from the mint without cherry-picking the
best coins before they put them up on eBay?
I used to. I don't submit coins for grading.
I no longer bother with mint sets now, though.
As a coin collector, I can not usually resist the urge to open
something that is sealed so that I take a look. But if I were a
seller, I could do it if there was more money in selling sets unopened
than there was in opening them and gambling that there was above
average coins that could be sold for even more money.
I have bought "sealed" 1957-1964 proof sets from various sellers and
I have generally been pleased with the quality of the coins. Maybe the
sets were opened and resealed or maybe they were never sealed by the
mint in the first place and just became sealed while in storage?
However, I was please with the quality of the coins for the price paid
and that is all that really mattered to me.
The only other sealed box I have bought second hand was a 2-roll set
of WV quarters. And I only bought it because it was the best price. I
actually opened it when I received it since I wanted to be sure it
contained both (P and D) rolls. I trusted the seller, but not the
mint. :) Of course with the coin roll sets the seller could only see
one side of the two end coins so there can really be no cherry
picking.